Sending a contract, a medical record, or payroll data as a plain email attachment means anyone who gets the file can read it. Adding an open password encrypts the PDF so it can't be viewed without the password — modern viewers all support it, nothing extra to install.
What the protection actually is
The Protect tool encrypts your file with AES-256, the current standard for PDF encryption. Without the password the contents are unreadable ciphertext — not hidden, encrypted. But the protection is only as good as the password: 'invoice2026' falls to a guessing script in seconds. Use a long passphrase, and share it over a different channel than the file (text the password, email the PDF).
Know the limits
A password stops the file being opened; it doesn't control what a recipient does after opening it — anyone with the password can print, copy, or re-save an unprotected copy. And there is no recovery: lose the password and the contents are gone for good, so store it somewhere safe.